No Apparent Cause

The aliens were microscopic. No one could see them. Most people, as a result, didn’t believe they existed, despite the findings of scientists and researchers.

The scientists believed that the aliens had come from somewhere in the vicinity of Proxima Centauri, and thought they were a highly aggressive species. The scientists also believed the aliens had extremely long life spans, and could easily adapt to life on Earth.

Most civilians didn’t believe any of this. They didn’t trust what the scientists were telling them - until humanity started mysteriously dying off, with no apparent cause.